Daylight in Schools and Early Research from 1999

You notice it right away, but you cannot explain it immediately. The moment you step onto a school grounds there is a mood in the air and inside the building it usually gets more intense. When I was working in Southeast Asia, I visited schools for my kids. It was going to be a longer stay. And it mattered to me that it would be a good time for them. So I wanted to see things for myself, especially since the region was completely new to me. Whether a school was private or public, you could tell instantly. The private ones put on a show right at the gate, the public ones felt natural from the start. But it was the buildings and the energy of the children in the spaces between them that set the tone. That is how I saw it, at least.

After a conversation about this and some research, I came across a study from 1999. Lisa Heschong. 21,000 students in California. The headline was banal: classrooms with more daylight produce better test scores. Every classroom has windows. That was not exactly new knowledge.

So I looked closer. At first the numbers fascinated me. What was measured, the wealth of detail was impressive. I am not a scientist, but I sensed there were gaps. Compared to others, the study was not even very detailed. But in my view it tackled the question in a fundamental way. From the mundane claim that light matters, all the way to why and to the measurement parameters.

I realized that there is much more behind a single research finding than that. It kept getting more complicated and it took me a while to work through it.

What exactly was measured?

Not “light” in general, but daylight. The light that comes through a window, not from a lamp or luminaire. An environmental factor with its own physical properties. The distinction matters. Artificial light and daylight do different things to the brain.

And what changed?

It was about test scores in math and reading. Something measurable that has consequences. Not “wellbeing” or “comfort.” Heschong compared classrooms with the most daylighting against those with the least.

By how much?

Students in the brightest classrooms scored 20% higher in math and 26% higher in reading than those in the darkest. That is definitely a difference you can see on a report card.

Why would light affect learning?

There is a physical pathway through which daylight affects mood and cognitive ability. Daylight hits specialized cells in the retina (ipRGCs) that have nothing to do with vision. They signal the brain’s master clock, the suprachiasmatic nucleus. That regulates cortisol and cortisol regulates alertness. The connection between light from a window and a test score runs through the endocrine system.

How reliable is this?

The study is cross-sectional. No one randomly assigned kids to dark classrooms. Probably the schools with more windows were also newer and better funded. The sample size of 21,000 helps. But the design is weak. In medicine there is a scale for this (GRADE) that makes exactly this visible. Not to devalue studies, but to show what additional evidence would strengthen them.

Later studies have partially closed the gaps. Boubekri et al. (2014, International Journal of Environmental Research and Public Health) quantified the effect of daylight on cognitive performance with lux values and exposure times. The direction held and the details got sharper. I want to write a separate essay on that.

Where was this studied?

These were elementary schools in California. Sunny climate and single-story buildings with wide windows. That is quite far from a basement office in the middle of Hamburg or a hospital in Oslo. Yet the finding is readily transferred there. The data does not support that.

Who was studied?

Children between 5 and 18 years old. Their internal clock works differently than in adults. The body follows a circadian rhythm, a daily cycle that controls when you are awake and when you are tired. In children, melatonin onset is later and the response to morning light is stronger. The finding is indirectly transferable to a 45-year-old knowledge worker, but you need to know that it is a leap.

How much daylight?

This is the most interesting part. The study compares “most daylighting” vs. “least daylighting.” It reports neither lux values nor window-to-wall ratios or hours of exposure. An architect cannot build “most daylighting.” They need a number. 300 lux at the work plane, minimum 2 hours of direct exposure, window area at least 20% of floor area. That data only appears in other studies.

When? And at what scale?

Measured during school hours, between eight and three. Morning light affects the body differently than afternoon light. And it was about a single classroom, not an entire building.

Can it backfire?

Yes. The same daylight that improves test scores can also cause discomfort. Too much direct sun on the work surface reflects off screens, causes glare and makes the room uncomfortably warm. Other studies show that above 400 lux of uncontrolled direct sun, complaints can increase sharply. So the benefit also has a limit.

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What started as a simple statement has many dimensions.

Most people stop at “daylight helps learning.” That is enough for a conversation. But if you are choosing a school for your own kids or designing one, you need the details.

The technology exists to do this systematically. Machines can pull in sources and compare them, find gaps and contradictions. What I did here with one finding needs to be done systematically for thousands of findings, so that those responsible for building design can actually do their work. What must come out of it is a structure that turns solid knowledge into planning solutions and above all architectural ones.

I have the impression that many people want to know more precisely why a room feels different. The scientific details do not make it simpler, but much clearer.